1. Field of the Invention
The present invention relates to a flyback system switching power source apparatus including a synchronous rectifying element.
2. Description of Related Art
As shown in FIG. 5, there is a flyback system switching power source apparatus 50 equipped with a synchronous rectifying element SW52 on the secondary side of a transformer T51. In such a power source apparatus 50, if the synchronous rectifying element SW52 on the secondary side is turned on in an on-period of a switching element SW51 on the primary side of the transformer T51, then the output terminals of the power source apparatus 50 are short-circuited through the secondary winding N52 of the transformer T51 and the synchronous rectifying element SW52. Accordingly, it is needed to control the synchronous rectifying element SW52 lest the switching element SW51 on the primary side and the synchronous rectifying element SW52 should be simultaneously turned on. Moreover, there is also the situation such that it is impossible to supply the control signals to be used in a primary side control circuit 51 directly to a secondary side control circuit 52 in order to insulate the primary side of the transformer T51 from the secondary side thereof. As a conventional control method of a synchronous rectifying element SW52, there is, for example, the method of detecting the polarity of the current flowing on the secondary side by a current transformer to switch the turning on and off of the synchronous rectifying element SW52 on the basis of the detected current polarity. Moreover, the following method was also proposed (see, for example, Japanese Patent Application Laid-Open Publication No. Hei 10-74936). The method inserted an inductor in series with the synchronous rectifying element, detected the polarity of the current flowing on the secondary side based on the voltage between both the ends of the synchronous rectifying element and the inductor, and turned off the synchronous rectifying element when the characteristic of a forward direction current was detected and turned on the synchronous rectifying element when the characteristic of a reverse direction current was detected.
The inventors of the present invention examined the possibility of realizing the detection of the off timing of the synchronous rectifying element SW52 in the flyback system switching power source apparatus 50 without adding any parts such as the current transformer and the inductor.
As the result of the examination, the method of measuring the drain-to-source voltage Vds52 of the synchronous rectifying element SW52 to detect a state Q1 in which the voltage Vds52 exceeded a predetermined threshold value Vth, as shown in FIG. 6 (Vds52), as the off timing was considered as the detection method of the off timing of the synchronous rectifying element SW52. Since a secondary side current I2 flowing through the synchronous rectifying element SW52 gradually decreased in an on-period of the synchronous rectifying element SW52, it was possible to detect the state Q1 in which the secondary side current I2 was less than the predetermined value as the off timing by detecting a voltage drop caused by the on-resistance of the synchronous rectifying element SW52.
On the other hand, in the case of adopting the detection method of the off timing mentioned above, there was the following case: ringing was generated in the drain voltage Vds52 of the synchronous rectifying element SW52 at the moment of the turning-on of the switching element SW51 on the primary side as shown in FIG. 6 (I2 and Vds52), and the drain voltage Vds temporarily exceeded the threshold voltage Vth owing to the ringing as shown at a time point Q2 in FIG. 6 (Vds52). Consequently, if any efforts were not made, the problem in which the synchronous rectifying element SW52 was erroneously turned off at the time point Q2 at which the drain voltage Vds exceeded the threshold voltage Vth owing to the ringing, after the synchronous rectifying element SW52 had been turned on was caused.
Generally, in order to settle such a problem, it is conceivable to set a minimum on-time To1 during which the synchronous rectifying element SW52 is forcibly being turned on in a period of from the timing at which the synchronous rectifying element SW52 has been turned on to the time when the ringing disappears as shown in a hatched part of FIG. 6 (Vgs52).
However, if the minimum on-time To1 mentioned above was set, it was found that the following disadvantageous case was sometimes caused. That is, as shown in a state Q3 in FIG. 6 (I2), if the output load of the switching power source apparatus 50 became light and the secondary side current I2 became less than a zero current in a period of the minimum on-time To1, then notwithstanding the drain voltage Vds52 of the synchronous rectifying element SW52 exceeded the threshold voltage Vth not by the influence of ringing but in a reflection of the secondary side current I2 at a time point Q1a in FIG. 6 (Vds52), the synchronous rectifying element SW52 was not turned off until the time point Q1b at which the minimum on-time To1 had elapsed. Consequently, a reverse flow was caused in the secondary side current I2 as shown in the state Q3 of FIG. 6 (I2) in that period.
Consequently, it was considered that it was needed to avoid the disadvantage mentioned above by detecting the magnitude of the output load of the switching power source apparatus 50 on the secondary side of the transformer T51.
Moreover, it was also considered that if it was able to surely judge the magnitude of the output load of the switching power source apparatus 50 on the secondary side of the transformer T51, other effective processing, such as changing the detection method of the on-off timing of the synchronous rectifying element SW52, was also able to be realized.
Although the magnitude of the output load can be judged by providing a current detecting resistance into the current pathway of the secondary side current I2 to measure the voltage between both the ends of the resistance, the method has the following problem. That is, if the resistance value of the current detecting resistance is made to be small, then accurate detection becomes hard, and if the resistance value is made to be large, then the loss by the resistance becomes large.